The present invention relates to a pressure sensitive adhesive sheet. More particularly, the present invention relates to a pressure sensitive adhesive sheet which, at the time of working of the back of an adherend having a surface whose irregular height differences are large, is preferably stuck to the adherend surface to thereby protect it during the working. The invention also relates to a method of use thereof.
In the step of grinding the back of a semiconductor wafer, the surface thereof having electrical circuits formed thereon is protected by a pressure sensitive adhesive sheet. The height differences of customary circuits attributed to electrode elements have ranged from about 5 to 20 xcexcm. For the wafer having these customary circuits formed thereon, the conventional surface protective sheets can fully protect the circuits without the occurrence of circuit breakage or wafer cracking and thus have been satisfactory.
However, in recent years, the method of mounting IC chips is being diversified. For example, the packaging 5 method in which an IC chip circuit surface is arranged down has been developed. In this packaging method, electrode elements protrude from the circuit surface and the height differences thereof are 30 xcexcm or more, occasionally greater than 100 xcexcm.
When the grinding of the back of the wafer having such protrudent electrode elements formed thereon is carried out while protecting the surface with the conventional surface protective sheet, it has been encountered that the back is deeply ground at portions corresponding the protrudent elements to thereby cause depressions (dimples) to occur on the back of the wafer. Further, it has been encountered that dimple portions crack resulting in the breakage of the wafer.
A similar problem is encountered with respect to ink (bad mark) provided for the marking of failure circuits after the inspection of the wafer circuits.
Protrudent portions formed on the surface of semiconductor wafers like those mentioned above are referred to bumps. For the semiconductor wafers having large bumps, measures have been taken, which include, for example, decreasing the hardness of the substrate film of the surface protective sheet and increasing the thickness of the pressure sensitive adhesive sheet. These measures, however, have been unsatisfactory, and the above problems have not yet been resolved.
The present invention has been made in view of the above state of the prior art. It is an object of the present invention to provide a pressure sensitive adhesive sheet which, at the time of working of the back of an adherend having a surface whose irregular height differences are large, is preferably stuck to the adherend surface to thereby protect it during the working. And another object of the present invention is to provide a method of use thereof.
The pressure sensitive adhesive sheet of the present invention comprises a substrate and, superimposed thereon, a pressure sensitive adhesive layer, this substrate exhibiting a maximum value of dynamic viscoelasticity tan xcex4 of at least 0.5 at a temperature ranging from xe2x88x925 to 80xc2x0 C.
It is preferred that the above substrate have a thickness and Young""s modulus whose product is in the range of 0.5 to 100 kg/cm.
The present invention is also concerned with a method of use of the above pressure sensitive adhesive sheet comprising sticking the pressure sensitive adhesive sheet to a surface of an adherend and working the adherend at its back while protecting the adherend surface by means of the pressure sensitive adhesive sheet.
This invention provides a surface protective pressure sensitive adhesive sheet which can precisely follow the irregularities of an adherend surface to thereby absorb the irregular height differences and which, at the time of grinding of the back of the adherend, enables accomplishing a smooth back grinding without being affected by the irregularities of the adherend surface.